A tank with its vent open to the atmosphere is
installed in the system above the highest radiator for
water expansion. The water level in the expansion tank
rises and falls, as the system is heated and cooled, and
the system is full of water and free from air at all times.
In the open-tank gravity hot-water heating system, the
expansion tank is installed on a riser directly above the
boiler, so the air liberated from the boiler water enters
the tank and is not retained in the system.
One-Pipe, Closed-Tank System
A one-pipe, closed-tank gravity hot-water
distribution system, as shown in tigure 4-65, is similar
to the one-pipe, open-tank gravity hot-water heating
system, except the expansion tank is a pneumatic
compression tank not open to the atmosphere. When
the water in a closed-tank system is heated, it. expands
into the pneumatic compression tank. This action
permits system operation at a much higher water
temperature, without boiling, than the temperature in
the one-pipe, open-tank gravity system. This also
results in higher heat emission from the radiators.
A gravity open-tank system with an average boiler
water temperature of 170°F has a radiator emission
rate of 150 Btu psi, whereas a gravity closed-tank
system with an average boiler water temperature of
190°F has a radiator emission of 180 Btu per square
foot (psf). Higher boiler water temperatures permit
higher temperature drops through the radiators;
consequently, smaller pipe sizes can be used. The
closed pneumatic compression system requires a relief
valve, usually set for the relief of water pressure over
30 psi, depending upon the height of the building. A
pressure-regulating valve automatically maintains the
system full of water. Installation of the radiators and
piping for an equivalent two-pipe, closed-tank gravity
upfeed or overhead downfeed system is the same as
that for the open system, except the sizes of both the
pipe and the radiators are uniform and can be smaller.
The open-tank system may have a reversed return main
that does not go directly back to the boiler.
It doubles back from the last radiator and parallels
the supply main back to the boiler entrance. The
reversed return system allows equal length of heating
circuits for al I radiators. Friction and temperature
losses for all radiators are nearly equal. In most cases,
the reversed return system involves no more piping
than other piping arrangements. With the correct size
of piping and radiator supply tappings, the reversed
return system provides even heat and circulation to all
radiators, even those near the end of the circuit.
Figure 4-65..A typical one-pipe, closed-tank distribution
Expansion in a Gravity Hot-Water
In the gravity and forced-circulation systems, open
and closed expansion tanks allow the water in the
distribution system to expand as the temperature rises.
An open tank must be mounted at the highest point in
the system; a closed tank can be located at any point. If
the air cushion leaks out of the closed expansion tank,
it fills with water. At times, you must recharge the tank
by draining part of the water out of the tank and
allowing air to -fill the space.
In the open system, an expansion tank open to the
atmosphere allows the system to expand. The open
system is normally designed to operate at the
maximum boiler temperature of 180°F. This gives an
average radiator temperature of 170°F or a radiator
output of 150 Btu psf. The closed system, in which the
expansion takes place against a cushion of air in the
tank closed against the atmosphere, can be operated at
temperatures above 212°F because the pressure built
up in the system prevents the water from boiling.
Radiator temperatures then become equal to those of
low-pressure steam systems.
When a hot-water system is first filled with water,
it is normally necessary to bleed the air out of the
system at the same time. You can remove the air by
opening an air vent on a radiator or by breaking a union
near the end of the line. The temperature of the water
distributed is from 150°F to 250°F. The higher
temperatures are used with the forced-circulation